1. Field of the Invention
The present invention relates protective coatings for optical fibers. More particularly, the present invention relates to radiation curable, secondary protective liquid coating composition for optical fibers formulated to have hydrophobic properties and optical fibers coated thereby.
2. Description of the Prior Art
For many years now, optical fibers made from drawn glass have been used as a reliable transmission medium in telecommunications cables. Glass optical fibers are widely used because they have the ability to carry large amounts of information over long distances. Glass fibers are inherently strong because the glass forming the fiber has an intrinsic strength on order of 3.times.10.sup.9 N/m.sup.2 (Pa). See, "Optical Fibers for Transmission", J. E. Midwinter, 1979, John Wiley and sons. The retention of such strength is largely dependent upon the application environment surrounding the optical fiber. For example, if the optical fiber is subjected to an abrasive environment, the abrasion, even with micron sized particles, produces microscopic flaws in the glass surface. These flaws propogate through the glass and eventually cause a fracture of the glass fiber. Glass optical fibers can also lose strength from contact with moisture and ions in solution. It is well known that hydrolysis of the surface bonds in silica glass occurs rapidly in the presence of water containing sodium ions. The hydrolysis of the surface bonds causes significant strength deterioration.
To protect the integrity and strength of glass optical fibers from environmentally caused degradation, one or more protective polymeric coatings have been applied to the outer surface, such as the cladding, of such optical fibers, shortly after the optical fibers have been drawn from a glass preform. One purpose of the coatings is to protect the surface of the optical fiber from mechanical scratches and abrasions typically caused by subsequent handling and use. Another purpose of the coatings is to protect the glass from exposure to moisture and ions in solution. The coating or coatings may also have some influence over the fiber's optical characteristics because the coatings are physically responsive to external mechanical forces and temperature.
The coating compositions applied to the optical fiber are typically viscous liquid, radiation curable compositions. Typically, the coating compositions are cured on the optical fiber by exposing the coating composition to ultraviolet radiation, electron beam radiation or ionizing radiation for a predetermined period of time deemed suitable for effective curing.
Fluorinated UV radiation curable secondary coatings have been provided over optical fibers to repel water and moisture. See generally U.S. Pat. Nos. 4,908,297, 5,298,291, and 5,567,794. The coatings disclosed in these patents are relatively expensive because the fluorinated raw materials which provide water repelling properties are relatively expensive. Accordingly, it is desirable to have a secondary coating having water repelling properties and good adhesion to the primary coating which is also relatively inexpensive.